No test can detect a CYA level lower than 30. If it has been two weeks since you last added stabilizer and you still don't have a reading, you can just assume that your reading is less than 30 and you need to add more. I would only add a dose that would add about 20ppm more, though. If your current reading is close to 30, that would put you at around 50ppm which would be a good reading.